F13 has six different meanings across six brands — a hot-water sensor fault on Vaillant and Protherm, control-board faults on Glow-worm and Saunier Duval, a heat-exchanger sensor on Ferroli, and a reset-limit lockout on Johnson & Starley warm-air units. Brand identification is the whole diagnosis here.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
F13 brand by brand
The F13 code on a Ferroli boiler indicates a problem with the heat exchanger NTC (Negative Temperature Coefficient) temperature probe — the thermistor that monitors the temperature of the heat exchanger and feeds that data back to the PCB. When the PCB receives an invalid, out-of-range, or absent signal from this sensor, it triggers F13 and shuts the boiler down as a precaution. On certain Ferroli model variants, F13 can also relate to a flue gas probe fault rather than the primary exchanger probe — the principle is the same: a sensor signal the PCB cannot trust. F13 is a blocking fault (as opposed to a lockout), meaning the boiler may recover automatically once the underlying problem is resolved, without needing a manual reset. However, if it keeps returning, professional diagnosis is needed.
Causes, fixes & costs for Ferroli F13 → · All Ferroli codes
The F13 code appears on Glow-Worm boilers when the main PCB detects a problem with its own connections or with the signals it receives from associated components such as temperature sensors and the wiring harness. In plain terms, something in the communication chain between the circuit board and the rest of the boiler has broken down — whether that is a loose connector, a degraded sensor, or a fault on the board itself. As a safety measure the boiler locks out and will not restart until the underlying issue is identified and corrected. The precise trigger can vary slightly across model ranges (CXI, HXI, SXI, Flexicom, Betacom, Energy), so if the standard checks below do not clear the fault, a Gas Safe engineer will need to carry out hands-on diagnostics.
Causes, fixes & costs for Glow-worm F13 → · All Glow-worm codes
F13 on a Johnson & Starley unit is a remote reset lockout: the appliance has locked out after too many reset attempts — typically via a room controller or repeated resets in a short window — and now refuses further remote resets. Controls count resets deliberately: each reset on a locked-out gas appliance re-attempts ignition, and unlimited retries against a genuine fault mean repeated gas admissions into an appliance that keeps failing to light. When the count is exceeded, the unit escalates to a state only clearable at the appliance itself, or by an engineer — which is precisely the point: something has caused the underlying lockouts, and it needs diagnosing rather than resetting again.
Causes, fixes & costs for Johnson & Starley F13 → · All Johnson & Starley codes
The F.13 code appears on Protherm boilers when the control system detects a problem with the domestic hot water (DHW) tank temperature sensor. The boiler's electronics continuously monitor signals from all connected sensors, and when the signal from the DHW sensor is absent, out of range, or erratic, the boiler flags F.13 and may shut down to protect itself. On the Panther Condens series, engineers are directed specifically to check for a sensor short-circuit or a fault at the X16 connector. On the Gepard range, F.12 and F.13 are closely related codes both pointing to the DHW tank sensor circuit. The exact presentation can vary slightly between Protherm model families, so always cross-reference with your specific installation manual — but in most cases F.13 points firmly at the DHW sensor or its associated wiring and PCB connection.
Causes, fixes & costs for Protherm F.13 → · All Protherm codes
The F13 fault code on a Saunier Duval boiler points to a problem with the main control board (PCB). The boiler's internal diagnostics have detected either a failure of the board itself, a fault in its wiring connections, or a component that communicates with it — such as the domestic hot water storage tank sensor. When F13 appears the boiler will typically lock out completely and refuse to fire until the underlying issue is resolved. Because Saunier Duval and Glow-Worm share a platform within the Vaillant Group, the same code is sometimes described on shared models as a 'PCB main connections fault', highlighting that wiring and connector integrity is just as important to check as the board itself. Older Saunier Duval models may also show F13 when the relay that switches between central heating and domestic hot water modes has failed, so the exact cause can vary by model age and specification.
Causes, fixes & costs for Saunier Duval F13 → · All Saunier Duval codes
The F.13 code tells you that the boiler's control board has detected a short circuit on the cylinder temperature sensor or warm start sensor circuit. These sensors use an NTC (Negative Temperature Coefficient) thermistor to feed live temperature data back to the PCB; when the signal collapses to a short, the boiler can no longer monitor cylinder or warm-start temperatures reliably and locks out to protect the system. F.13 most commonly appears on Vaillant combination boilers fitted with an actoSTOR unit or a warm start function, and it very frequently appears alongside fault code F.91 — if you see both codes, the cylinder charging sensor circuit is the prime suspect. There are no direct gas-safety implications, but the fault will disrupt hot water and heating operation until it is resolved.
Causes, fixes & costs for Vaillant F.13 → · All Vaillant codes
General guidance only — not a substitute for professional advice, and not certified by a registered engineer.
Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.
